1. Field of the Invention
This invention relates to an apparatus and a method for detecting charge and, more particularly, to means for accurately detecting the charged potential of an article to be conveyed such as a semiconductor substrate, etc.
2. Description of the Prior Art
FIG. 11 shows a conventional charge detecting apparatus for ion implantation. The charge detecting apparatus is composed of a detecting electrode 102, a field effect transistor 103, an amplifier 104, etc. contained in a case 101. An opening 105 is perforated at the front face of the detecting electrode 102. A semiconductor substrate 107 on a rotary disk 106 upon which ion implanting has been done is moved to the front of the opening 105 to induce charge at the detecting electrode, thereby varying the gate electric field of the field effect transistor 103. Thus, an output responsive to the variation in the quantity of the charge indicates the potential of the semiconductor substrate 107.
Since the surface potential of the rotary disk 106 on which the semiconductor substrate 107 is placed is zero in the above-described means, the potential of the semiconductor substrate 107 is equal to the detected value of the potential of the substrate 107 when the rotary disk 106 passes the front face of the detecting electrode 102.
However, a case that sometimes a case that the surface of the rotary disk 106 has deteriorated and the surface potential of the rotary disk 106 is not zero due to the film on the rotary disk 106. Thus, there arises a problem that the potential of the semiconductor substrate 107 cannot be accurately measured.
FIG. 12 shows a conventional apparatus for implanting ions onto a semiconductor substrate. Semiconductor substrates 201 are aligned on the surface of a rotary disk 200. An ion beam generator 202 is opposed to the rotary disk 200. When an ion beam irradiated from the ion beam generator 202 is radiated onto the semiconductor substrate 201, electrons are ejected from the semiconductor substrate 201. The semiconductor substrate 201 is thus charged causing the amount of ions implanted to be unpredictable or causing other problems, such as an insulator breakdown, etc. Therefore, in such an ion implanting step, the charging state of the semiconductor substrate is generally monitored.
Accordingly, a detecting electrode 203 is provided at a position opposed to the semiconductor substrate 201 on the rotary disk 200, a charge detector 204 for measuring the voltage of the detecting electrode 203 is provided, and the neutral amount of the charge on the surface of the semiconductor substrate 201 is determined according to the output signal of the charge detector 204.
It generally takes several milliseconds from when the ion beam is irradiated from the ion beam generator 202 to the semiconductor substrate 201 until the semiconductor substrate 201 passes the front of the detecting electrode 203 as described above. The charge attenuating characteristic during this period is different depending upon the type of semiconductor substrate 201 used. However, since the conventional above-described means uniformly determines the neutral amount of the charge of the semiconductor substrate in response to the voltage of the detecting electrode irrespective of the different charge attenuating characteristic depending upon the type of the substrate 201 used, there arises a problem that a suitable neutralization remedy cannot be employed in response to the type of the substrate.